The present invention relates to a scanning optical system typically employed in a laser beam printer or the like.
In the scanning optical system for the laser beam printer, a laser beam emitted by a laser diode is deflected by a polygonal mirror to scan within a predetermined angular range. The scanning beam passes through an fθ lens, which converges the beam to form a scanning beam spot on a photoconductive surface. As the polygonal mirror rotates, the beam spot moves on the photoconductive surface. By ON/OFF modulating the beam spot as it moves, a electrostatic latent image is formed on the photoconductive drum. Hereinafter, a direction, on the photoconductive surface, in which the beam spot moves as the polygonal mirror rotates is referred to as a main scanning direction, while a direction perpendicular to the main scanning direction, on the photoconductive surface, is referred to as an auxiliary scanning direction.
Further, shape and direction of power of each optical element is described with reference to directions on the photoconductive surface. Further, a plane perpendicular to a rotation axis of the polygonal mirror and including an optical axis of the scanning lens is defined as a main scanning plane.
Sometimes, a multi-beam scanning optical system is configured such that a plurality of beams are deflected simultaneously by a single polygonal mirror. With such a configuration, since a single polygonal mirror is used as a deflector for each of the plurality of beams, the number of optical elements can be decreased, and a room for such elements can be reduced. If the plurality of the beams are respectively inclined in the auxiliary scanning direction, and are incident on substantially the same point on the polygonal mirror, the thickness of the polygonal mirror can be reduced, which reduces a manufacturing cost of the polygonal mirror.
However, if a beam is incident on the polygonal mirror as inclined in the auxiliary direction, a bow occurs, that is, a scanning line, which is defined as a locus of a beam on a surface to be scanned, curves. The bow deteriorates an imaging accuracy and should be suppressed particularly for a high-resolution scanning system.